The concentration of one or more fluid substances (i.e. gases or liquids) within a sample can be determined via optical absorption spectroscopy, by passing light through the sample and detecting the optical absorption characteristics of those substances. The term “light” as used herein is intended to encompass the infrared and ultraviolet wavebands, as well as visible light. The term “waveband” refers to each of the three broad spectra of light comprising ultraviolet, visible and infrared light having frequency ranges respectively of 30000-790 THz, 790-400 THz and 400-0.3 THz, and also to subdivisions of those spectra such as the near infrared (400-120 THz).
The amount of light absorbed by the substance and therefore the sensitivity of the method depends on the concentration of the substance and the path length of light through the substance. In gases, the concentration in terms of molecules per unit volume is generally much lower than in liquids or solids and therefore the path length of the light through the sample must be correspondingly higher. For example, the required path length is typically between about 2 m and 100 m for gas mixtures containing low concentrations of the target gases, such as atmospheric pollutants. This large path length can be achieved either by placing the light source and the detector far apart or by reflecting the light backwards and forwards through a sample in an optical cell so that it passes through the sample numerous times before reaching the detector.
The utilisation of a multi-pass optical cell can therefore provide a long path length in an apparatus having a compact form. An example of a multi-pass optical cell is the White cell. The basic White cell is a multi-reflection system conceived by J. U. White and initially published in “Long Optical Paths of Large Aperture”, Journal of the Optical Society of America, May 1942.
A typical White cell 1 consists of three concave mirrors 2, 4, 4′ of identical radius of curvature, the basic configuration of which can be seen in FIGS. 1 to 3. The front (or field) mirror 2 faces the two side-by-side back (or objective) mirrors 4, 4′, the distance between the two sets of mirrors being twice their focal length. Light from a source 6 at a point F0 adjacent one edge of the front mirror is focused by the first back mirror 4 onto the surface of the front mirror 2 at point F1. The front mirror 2 is oriented such that it reflects the light towards the second back mirror 4′, which refocuses the light at point F2 on the front mirror 2. This light is then refocused by the first back mirror 4 at point F3, and so on thus forming a set of foci F1, F2, F3, . . . across the surface of the front mirror 2. Eventually, after n passes, the light reflected by the second back mirror 4′ falls off one side of the front mirror 2 at focal point Fn and is collected by a detector 8. This light is then analysed by a spectrograph 10 to detect the optical absorption spectra of the substances through which the light has passed. In this example, the light source 6 and the detector 8 are each provided with a lens 12, 13 for focussing the light.
The second back mirror 4′ can usually be rotated about its vertical axis as illustrated by arrow A in order to adjust how many times the light is reflected between the front and back mirrors before it falls off the side of the front mirror 2. This is referred to as “yaw adjustment”. By adjusting the yaw, the pathlength of light through the sample can be controlled. One or both back mirrors 4, 4′ may also be adjustable about a horizontal axis to adjust the plane in which the light is reflected. This is known as “pitch adjustment”.
The White cell 1 is normally provided with a housing 14 as shown in FIG. 2, which contains the fluid sample. This housing 14 may be provided with entrance and exit windows 15, 17 for transmitting light to and from the cell 1.
In its most basic form, the light reflected within the White cell 1 stays entirely in one plane, which is referred to herein as the primary optical plane (this is commonly the horizontal plane). However, in practice the pitch of the two smaller objective mirrors 4, 4′ is usually adjusted such that they are tilted in opposite directions in the vertical plane. The result of this is that the pattern on spots across the field mirror 2 is staggered across two separate rows as shown in FIG. 3. One of the rows has an even number of spots F1, F3, F5, etc. and the other row has an odd number of spots F2, F4, F6, etc. In this example the row with an odd number of spots is in the same plane as the entrance and exit windows 15, 17 and the source 6 and detector 8.
As will be apparent from the description above, the light from the source 6 is repeatedly refocused such that the effects of divergence over a long path length are minimised. Such divergent effects are typical from non-point sources of light and non-ideal collimation assemblies: this makes the White cell particularly useful for arc-based lamps. The White cell is the preferred multi-pass optical cell, although many practical alternatives exist such as Herriot cells, passive resonators, integrating spheres, etc.
Multi-pass optical cells are generally designed for use with electromagnetic radiation in a specific waveband, for example ultraviolet, visible or infrared. The chosen waveband dictates the choice of materials used for various optical components of the optical cell, such as the mirrors, entrance and exit windows and transmission optics (for example optical fibres). However, materials that are suitable for one waveband are often unsuitable for another waveband. For example, fused silica used as a window material is almost transparent to UV but virtually opaque to IR at certain wavelengths, whereas for a mirror material, silver has a high reflectance for visible and IR, but a very low reflectance at certain UV wavelengths. Therefore, multi-pass optical cells are generally designed for use with electromagnetic radiation in a specific waveband and cannot be used with radiation of other wavebands.
Of the various fluid substances that may be of interest, some may absorb significantly only in one waveband such as UV, whereas others may absorb significantly only in another waveband such as IR. If both types of substance are of interest, the inability of existing significant disadvantage. Yet other substances may absorb in both the UV and IR bands: in this case it may be beneficial to detect absorption in both bands for greater accuracy or sensitivity. However, this cannot be achieved with a conventional multi-pass cell.
U.S. Pat. No. 7,288,770 discloses a portable air monitoring system using UV spectroscopy capable of detecting chemicals in the open atmosphere or in a sample of air which is introduced into a chamber. The system enhances its sensitivity and accuracy by collecting a full spectrum of data points and by using multiple mirrors to increase the beam path in a closed-path length. The accuracy of these methods is good but not sufficient to fulfill the requirements that are present today, such as forming time-varying or real-time representative maps of pollution levels. Furthermore, this method does not allow for detection in multiple light wavebands.
U.S. Pat. No. 4,969,156 discloses a system including the adjustment of the second mirror to allow for calibration through overlapping laser pulses. The system does not provide for the determination of simultaneous absorptions across various frequencies.
U.S. Pat. No. 5,838,008 discloses the use of a White cell for the determination of gas concentrations via FTIR (Fourier transfer infrared spectroscopy). The method does not allow for the simultaneous detection of different wavebands of light.